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In this study, diagnostic equations are proposed to quantitatively evaluate meridional overturning circulation (MOC) simulated in ocean general circulation models (OGCMs). Applicability of the equations is illustrated by revisiting the MOC simulated in an idealized ocean. The simulations with surface differential heating/cooling show that, for certain horizontal distribution of vertical diffusivity, the stronger vertical mixing does not intensify the MOC while it makes the deeper water less dense. This result, which is in marked contrast to the widely accepted idea that the stronger vertical mixing promotes upwelling and intensifies the MOC by making the deeper water less dense, was investigated using the diagnostic equations. It was found that geostrophy dominates the MOC, and the geostrophic flow normal to lateral boundaries induced intense upwelling/downwelling along the boundaries. These results indicate that the primary role played by the vertical mixing on the large-scale MOC is to change hydrostatic pressure fields (geostrophic flow fields), rather than to promote upwelling. The simulation with localized cooling on the other hand showed that the ageostrophic flows significantly contribute to small-scale features of the MOC, while the geostrophic flows determine large-scale structure of the MOC. The proposed equations will thus be useful to quantitatively diagnose the MOC dynamics in realistic OGCMs.

The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26 degrees N-a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high-resolution models. Furthermore, we will describe how high-resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.

The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N ‐ a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high‐resolution models. Furthermore, we will describe how high‐resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.